Satellite Positioning Systems (SPS) are used in many areas, such as providing accurate timing, as well as accurate navigation and positioning for vehicles such as aircraft, cars, ships, and the like. Satellite Positioning systems can be classified as either a Global Navigation Satellite System (GNSS), such as the Global Positioning System (GPS), the Galileo system, the GLONASS system, and the Beidou system which provide global coverage, or as a regional system which just provides regional coverage, such as the Indian Regional Navigation Satellite System or IRNSS with an operational name of NAVIC (Sailor or Navigator in Hindi) system. In this description, however, we often use the term GNSS to refer to any satellite positioning system, whether global or regional, unless the context clearly requires otherwise, and hence the terms Satellite Positioning System (SPS) and Global Navigation Satellite System (GNSS) are often used herein interchangeably.
Whilst many GNSS/SPS systems will provide native accuracy of a few meters, improved accuracy can be obtained by using differential techniques which make use of a surveyed reference GNSS receiver, which compares the position obtained by its GNSS receiver to its known surveyed position, and then broadcasts position correction data to other ground stations, which then use the position correction data in combination with their own position fixes from the GNSS system in order to obtain a more accurate position fix. Such dGNSS systems (referred to herein more generally as dSPS systems to also include where the SPS is a regional system) often have only a single surveyed reference station, and the accuracy of the position fixes obtained by the rover stations decreases with distance from the fixed survey station.
More recently, networks of surveyed reference GNSS receiver stations have been installed across geographical regions. For example, in the UK the “OS Net” network of precisely located GNSS receiver stations provides a national network of reference GNSS stations which can be used to improve the accuracy of position fixes of other mobile stations. In practice, the reference stations at the precisely surveyed geographic locations may use real time kinematic (RTK) satellite navigation techniques, where the phase of the GNSS signal's carrier wave is measured, thereby allowing highly accurate positioning, typically to one to two centimetres accuracy. FIG. 1 illustrates such a network of reference stations, wherein a plurality of reference stations 12 are distributed across a geographic region 1, at precisely surveyed and known geographic locations. The reference stations 12 may use dGNSS or RTK techniques to obtain position fixes, which fixes are then fed back to a central server 30, which collates the position fix data from each reference station 12, and compares it to the known geographic location of each reference station 12, to thereby allow position fix correction data to be computed. A mobile station 20 typically subscribes to a service which allows access to the position fix correction data calculated by the network RTK server 30, and when it is undertaking a position fix, it contacts the network RTK server 30, and receives the position fix correction data. The position fix correction data is then used by the mobile terminal 20 to improve the accuracy of its own position fix that it takes using signals transmitted by the constellation of GNSS satellite 10. The GNSS satellite constellation 10 may be any of satellites belonging to the Galileo, GPS, or GLONASS GNSS, or Beidou or instead of being global may be any of the other regional SPS constellations, such as IRNSS.
With such an arrangement, accurate positioning across the geographical area 1 can be obtained, to sub-meter, centimetre, or even subcentimetre accuracy, depending on the length of time observed and the capability of the user's GNSS equipment. Such accuracy can be very important for surveying applications, as well as for machinery control (e.g. farm machinery), Unmanned Aerial Vehicle operations, asset tracking, and the like.
Another issue that arises in the use of GNSS systems is whether or not accurate GNSS signals are being received, such that an accurate position fix can be obtained therefrom. In this respect, GNSS signals can be subject to spoofing and jamming, in order to introduce errors into position fixes obtained therefrom. In order to address this problem, several different solutions have been proposed previously, two of which are shown respectively in FIGS. 2 and 3. These are both described next.
Within GNSS systems it is common for there to be an encrypted channel for government and military use, which is heavily encrypted and therefore resistant to jamming and spoofing thereof. However, because the channel is heavily encrypted, hardware security modules are required at the receiver that contain decryption key material therein. Such hardware security modules are cryptographic controlled items, which are not freely available to the public. However, by decrypting the encrypted navigation channels in GNSS systems, it is possible to authenticate a position fix obtained from the open channels. It would therefore be useful for such authentication techniques using the encrypted navigation channels to be more widely available, without compromising the cryptographic controlled items such as the hardware security modules which contain the decryption key material. Two previous approaches in relation to the Galileo public regulated service (PRS), which is the government authorised-user service which makes use of cryptographically generated navigation signals, and which can be decrypted via security controlled hardware security modules, are described next.
Both FIGS. 2 and 3 present server-based Galileo PRS services, where the PRS decryption key is held securely by a government trusted party, which provides a decryption and authentication service to public users. FIG. 2 illustrates a first system, referred to as the ASPIRE (Affordable Secured PURSUIT PRS Integrated REceivers) system, which is described in further detail in WO2012/007720 in the name of Thales UK. In this system, a mobile unit 20 receives GNSS signals from the GNSS constellation 10, in this case the Galileo constellation. The mobile station 20 then performs a position fix based on the received signals, and at the same time the open service GNSS as well as Galileo PRS signals RF signals are captured, screened and conditioned before forwarding to a remote server for processing. The open service and encrypted PRS signals are then transmitted to a PRS decryption server 42, which is located at a government-trusted third party 40. The PRS decryption server 42 has access to the PRS decryption key 44, and is able to decrypt the received PRS signals from the mobile station 20, and authenticate them to confirm that they are genuine. Authentication of the open service signal against the PRS signal is completed and an authentication signal is then transmitted back to the mobile station 20, to confirm that the received PRS signals that were sent from the mobile station 20 to the PRS decryption server 42 are genuine. This operation gives the mobile station 20 comfort that if the PRS signals have not been tampered with, or spoofed, and hence are genuine, then it is likely that the open signals upon which it has based its position fix are also likely to be genuine, and hence the position fix can be relied upon.
In the above, it is only necessary for the mobile station 20 to demodulate and record the PRS signal from one satellite 10 in the Galileo constellation, as provided that a single PRS signal can be decrypted, then the position fix using the open Galileo signals can be authenticated. However, it is also possible that multiple PRS signals from the Galileo constellation in view could be recorded, in which case those signals can all be transmitted to the server 42 via the mobile station 20, where they are then decrypted and, because there are plural such signals, a PRS based position fix can then be obtained. This PRS based position fix can then also be communicated back to the mobile station 20. Thus, the ASPIRE system allows a mobile station 20 to either authenticate its own position fix based upon the open Galileo signals using the PRS signals as sent to a trusted decryption server, or for a PRS position fix to be obtained, where the mobile station 20 records and forwards multiple PRS signals from multiple satellites in the Galileo constellation to the trusted decryption server. In both instances, however, the PRS decryption key 44 is kept securely with the trusted party 40, and is not exposed to the mobile station user 20.
FIG. 3 illustrates a further prior art system, known as the PROSPA (PRs/Open Service Positioning and Authentication) system. Further details of the PROSPA system are available in EP2799908A1, owned by Nottingham Scientific Limited. The PROSPA system is a broadcast system that transmits a so-called “snippet message” for a given point in time to mobile subscriber terminals 20. The snippet message contains enough data to permit the mobile terminals 20 to correlate the GNSS encyrpted signals at that time. Snippet messages are broadcast to user terminals via a secure communications channel and once received, the snippet allows the mobile terminals 20 to correlate at least part of the received PRS signal with the result that if the PRS signal can be correlated, it is then known that the signals have not been tampered with, and hence the fix obtained from the Galileo open signal can be authenticated. The PROSPA system therefore requires less messaging than the ASPIRE system, in that it is a broadcast system wherein the snippet message is broadcast to subscriber terminals and then used at this subscriber terminal for PRS correlation, but it does require additional processing to be performed at the subscriber terminal, and hence the subscriber terminal 20 has to be more complex than the terminal 20 used in the ASPIRE system. In addition, there is also the need for a secure channel between the snippet generator 52 and the mobile terminals 20 over which the snippet message can be passed.